Method of making iron oxide pigments



Patented Apr. 7, 1953 METHOD OF MAKING IRON oxnin PIGMENTS Geoi'g'eE Noponen, White Bear, Minni, assignor to Minnesota. Mining & Manufacturing; Conipan a St. Paul, Minn a corporatioii of Dela ware in viewing; Application December-1'9; 947; Serial No. 792,867

, I This invention relates to the manufacture of l ironnxide, particularly for useasa'pigment; for" use as arouge for polishing-glass; and for Other 118655 This application" is a" continuation impartof my "copendi-ng application Serial No:"449;025; filed J une-29; 1942; nowab'andonedr I The-principaLobjectof the invention isthe production of iron-oxide pigments of high-quality;v particularly having a high tinting strength and high-chroma masstone."" Theinventi'on has par= ti-oular utility" in improving' the iron oxide products obtained in methodsin which" substantial portions of the iron content of iron-ore-(ineluding naturally occurring orefpyrit'es' "cinder" and "such like) are convertedto ferricsulfateand then cal cined to produce -'the*desired iron oxide,- a's'well as in theproduction-of"iron*oxide pigments from copperase U A v I I In calcining iron sulfate to'producearediron oxide pigment, the-ironsulfate is" customarily calcined foran hour ormoreat temperatures of approximately 1 1100-- F. to aproximately 1800 F. in a-roasting furnace-either of therota-ry type; continuousbelt; semi-multie type: closed; mufile type-or reverberatory type: Inany oftheseroast v ing furnaces, "and *inview-of-the=relatively high temperatures employed; it-is unavoidably cus tomary in commercial practices, if the sulfate is" substantially completely -decomposed,="to overburn the iron oxide; with the resultthat*the'ironnxide produced contains considerableamounts of ma-- terialof poor pigment qualityin" that it has a low tintingstrength and low' chroma'mass'toner Thesintersproduced in the customary com mercial practice, which do yield some" ironoxide of pigment qualities; contain-undecomposediron sulfate and accordingly mustbe washed to -re-' move the iron sulfate and dried before'the -mate-- rial can be employed as apigment; Arr-important object of the present invention is the development' of a calcining: method in which tl1e=iron sulfate is. substantially i completely? decomposed without-- overburning the 1 iron oxide i-in commercial :scaleoperations, and 1 accordingly the calcined product need not be Washed and dried astiscustomaryr-in1 the commercial practicesknown to-the art. A

' further object ofthe inventionisthe reduction of the fuel costs in the calcination of iron sulfate. A fu rther important object of the-invention is the it development of a commercial; calcining method that produces a neutral or sulfate-free sinter with a minimum of heat input, wherebypig'ments of maximum brightness and maximumv chroma are produced. I increasing the rate of calcii'iatiorl another object of myinvention.

liesirl thered'iictioh -0th iIi the' 'time 'and'c'o'st of V calc'inati'o'nl' In the cornmercial'calcinatio'n of copp'ei'astit' isc'ustom'agr" 'tof'first' dry the material, before it' is roastedto prodiicethe iron oxide pigment; since it has b'nffolmd thatif tli'e' COBB-Bras is not first dried under controlled conditions,

the material" is not first subjected tojth'e'f ns Ara mp rtant feature-or the invention is the fact that" the ga 's evolved in the "calci'nation of iron sulfat'sjto if'oh' oxid'iiihlaccordnce'With phur dioxide" (soar to" sulphur ftri'oxid (s03) than is'obtainedTin calcining' iron sulfate accordmg to the 'methods'knovifn to th'eart. (In the absence'bf' atmospheric" oxygen: the gas evolved contains sub'stant'iallyho The unavoidable presence'bf'ai'r,- inanyjcoiiilnercialcalcining oper atib'n; in: cont'at fw ith the bed of matefial being calcined," nsa'iilyislllfis in ie-Oxidat ion Of "a Small p bi'ftiofi 0f the S62 136801;. Th'e alhount 0f the latter 'awevr, is kept to a minimu in my improved process.) 7 This restorers of articunr importance whn tnerecovery" of sulphur gases ,as sulphuric" 'aci-dis contemplated," since in the contact sulphuric aegiq'procsstne sulphur dioxide must be" fre'eanfsulphur' trioxid before it is suitable for the acid'reovery plant? The 'tW'd lIioSt cbi'hll'lilll irorl sulfates are ferrous a sulfate f Fesopi andj ferrji enate L (Fez (sons) In calcining'fer'rous sulfate and ferric sulfate,

1 9 of th abqvereecti e ar ende h fie nd tq iraf esttnisupr lqf e t to j m -v o 1 "the;F WFQI LS a Qi F iQ -QQFQQ e. t as b eer n i' q "be Pr tica im o b to u ly the a q it i 'siii e l al new. req ii mp teill v sins wj i sz t erbv e the on xide, E6203 m fin he ear s ag f the omn si qr f f hence 51 E5 q oe iie has P001" h EQQQQWWW emit ad 3 3- tionin the'sulfate sin teras the calcinatioirpro gresses on qui n hi h. t m ratur a d pr longed periods of heating to satisfy the heat requirements 9f h "e r re i q sf;

I have found that an exothermicreaction,

which serves to promote the foregoing reactions and to accomplish the aforementioned objects, can be carried out simultaneously with the above reactions in the iron sulfate mass by having present in the mass a stoichiometric amount of a reagent such as carbon in one or more of its various forms. It has been found that carbon reacts with the sulphur trioxide, formed upon decomposition of iron sulfate, and gives off considerable quantities of heat due to the exothermic nature of the reaction, and thus the requirements of external heat and the time of calcination are materially reduced. The addition of carbon to the iron sulfate mass prior to calcination, produces the following reaction when the mass is heated during calcination:

This reaction is exothermic and supplies an additional and internal source of heat adapted to aid in carrying the above decomposition reactions of ferrous sulfate and ferric sulfate substantially to completion. A further important advantage of my improved method of calcining iron sulfate is the fact that the heat given off by the reaction between carbon and sulphur trioxide is supplied at a time when it is needed to decompose the remaining iron sulfate and since the heat is supplied within the sinter, it thus promotes the decomposition of the iron sulfate to ferric oxide without the necessity of heating excessively the poorly conductive iron oxide first formed. Thus less external heat is required to carry out the calcination, and the overburning of ferric oxide, which is common in customary commercial calcination of iron sulfate and tends to produce considerable quantities of iron oxide of a poor pigment quality, is eliminated.

The improved method may be employed in calcining in any of the well-known types of roasting furnaces, such as those previously described. The carbon is added in finely divided form and may comprise powdered coal, powdered coke, carbon black, starch, molasses, sugar, charcoal, sawdust and the like, and since most of these materials are relatively inexpensive, a reduction in the total materials cost of calcination is obtained due to the lower fuel costs resulting from my improvement.

I have found that the addition of carbon in stoichiometric amounts based on the sulfate content of the iron sulfate starting material, in accordance with the chemical reaction hereinbefore indicated, provides for the burning of the iron sulfate in commercial operations to a neutral or sulfate-free, but not overburned, sinter. For example, one pound of powdered coal or coke, or other forms of carbon, is added to 100 pounds of copperas (FeSO4'7H20). In calcin ing copperas, the carbon is added to and well mixed, as by tumbling, with the granular iron sulfate before the external heat of calcination is applied.

In calcining ferric sulfate produced by converting iron ore, the carbonaceous material, in finely divided form, is preferably added to and thoroughly mixed with the ground iron ore prior to its conversion to ferric sulfate, although if desired, it may be mixed with the granular ferric sulfate after conversion. Approximately 4.5 pounds of carbon is added to the ore for each 100 pounds of ferric sulfate (Fez(SO4)3) to be produced in the conversion. The employment of carbon in excess of the stoichiometric proportions is to be avoided, since the excess carbon will act as a reducing agent on the iron oxide (Fezoa) formed. On the other hand, a deficiency in the amount of carbon requires either that the mixture be incompletely burned, thus leaving an acid sinter Which must be washed, or that portions of the mixture be overburned in order to make certainof the complete conversion of iron sulfate to iron oxide.

Acidity of the pigment product is determined by titrating a water extract of a sample with a standard alkali and expressing the result as percent H2804. My product can easily be controlled within an acidity of about 1 percent and can ordinarily be controlled within an acidity of less than of 1 percent. Pigments of such low acidity are regarded as neutral, as herein discussed; and such low acidity is of no real consequence in respect to wide spread commercial uses of my pigment product, such as in paints, ceramic coatings, floor tile, etc.

As an example of the utility of my improvement, it may be considered in connection with a method of producing red. iron oxide pigments from iron ore as a starting material, in which upwardly of approximately 70% of the iron content of the ore is first converted to ferric sulfate to produce subsequently a pigment of high quality. One example of such a method comprises mixing 200 lbs. of ground iron ore and approximately 16.5 lbs. of carbon with approximately 280 lbs. of concentrated sulphuric acid of 96% H2804 content in an insulated internal mixer. Approximately 150 lbs. of water are then added and the reaction between the iron oxide and sulphuric acid is started by the heat of dilution of the acid and carried on by the heat given off by the exothermic reaction until approximately of the iron of the ore is converted to ferric sulfate. Then the viscous fluid mixture is discharged into a suitable container where it solidifies to a granular form which can be conveniently handled during the roasting process, or, more preferably, the mobile but viscous mixture (with the stoichiometric proportion of carbon present) is formed into shaped pieces having a uniform small dimension or diameter, a by extrusion, as described more in detail in the copending application of Johnson and Bauman Serial No. 479,370, filed March 16, 1943, now abandoned, continuation-in-part Serial No. 20,357, filed April 10, 1948, now U. S. Letters Patent 2,541,068, issued February 13, 1951. The extrusions may, for example, be about inch in diameter and about 1 /2 to 2 inches in length. The carbonaceous material, in one or more of the forms mentioned, and in substantially stoichiometric parts as indicated, may be added before the ore is converted, or it may be added after the converted material has cooled. The material is then placed in a roasting furnace of one of the types described, and calcined to convert the ferric sulfate to iron oxide.

In my calcining steps I have employed a rotary kiln 60 feet long having an inside diameter of the kiln shell of 6 feet and an inside diameter of the kiln (inside of the kiln blocks) of 4 feet 9 inches. The pitch of the kiln was set at of an inch per foot. At one end of the kiln (the higher end) provision is made for the introduction of a charge of pellets or extrusions of a mixture of iron sulfate and carbon, and at the same end provision is made for withdrawing gases from the kiln through a closed system. At the other end of the kiln, i. e., the discharge end of the kiln, provision is made for discharging the calcined sinter, that is, the pigment product, and at the same end of the kiln a fuel oil burner is mounted for the direct firing of the kiln. fThe fuel oil burner employed atomizing type of burner, such as the 6 inch burner designed for-use with heavy fuel oil as made by North American Manufacturing Company, Cleveland, Ohio.

I Particles or extrusions comprising a'mix-ture of iron sulfate and carbon are first passed through 1a rotary' drier where they are heated toabout 200 or 300 F., so as to eliminate much of the moisture and to yield a sensibly dry material. These dried particles orextrusions (for example about 'inch indie-meter and l to 2 inches length) are'fed directly into-pthe rotary kiln at '4' About 3,000 to LO'OQ cubic feet per minute of gases (measured at about atmospheric pressure and about 90 F'.) is withdrawn from the opposite end of the kiln. Thesint'er or calcined product is discharged continuously from the discharge end of the kiln, at the rate of 1800 pounds per hour, and the discharge temperature of the calcined product is aloou-t'1350 to 1800'F'. The exact dis-- charge temperature of the product is held constant when the desired shade is produced. However at different times, due to varying calcining conditions, the discharge temperatures may differ even though the same shade .of-color is being made, usually about 1600" to 1800" The timeofpassage of the extrusions through the kiln is about 30-40 minutes. The product from the kiln is customarily ground to reduce agglomerationof pigment particles. The product is a red iron oxide pigment.

Let us assume that, because of slight variations within the specific conditions named, the

pigment product, although burned sufficiently neutral, e. g. to within an acidity of about 1 percent, is not as orange red in hue as is desired. By cutting down the rate at which fuel oil is'suppliedto the burner, the product will be changed toga more orange redshade. If the rate of feed of the fuel oilis cut down too far, keeping the other conditions iconsliantfthenithe result will be thatthe sinter leaving the kiln will not be burned neutral; so there is a" practical limit on how far the shade ofthe product can be controlledby simply controlling the heat input to the "kiln. If the'product emerging from the discharge end of the kiln is the :desired shade of color, but is still acidic, .i. e. is not burned substantially neutral, then by reducing the rate of rotation of the kiln and/or reducing the rate at which iron sulfate particles are fed to the kiln, with some further possible decrease in the rate the fuel oil is fed, the calcined product leaving the kiln can be brought to substantial neutrality, while still maintaining the color of the product at the desired shade.

in the operation is a' gun type, low pressure trusions are passing through'the kiln, it is selfevident'that the pounds per hour of production will be low for the equipment employed. The foregoing specific example involves a depth of particlesin the kiln "whichis in the preferred range. As set out hereinabovait is desired to have the carbon in the iron sulfate charge present in stoichiometrie proportions based ion the sulfate content of the iron sulfate, in accordance with the chemical equations shown hereinabove.

However in practical operation; where a slurry of iron ore and carbon '(such as powdered petroleum coke) is beingbroug-ht into contact with sulfuric acid at 'oneend of a reactor and'then passed therethrough and the iron sulfate productthen'formed into extrusions, one can ordinar' ily only approximate exact stoichiometric proportions. The control of the proportion of carbon should nonetheless never deviate -more than 8' percent, or so, from the stoichiometric proportion and it is highlydesirable, and normally quite feasibie to control it within about 5 or 6 percent of the exact stoichiometric proportion, or even closer, and approximations to within 2 or 3 per cent of the exact 'stoichiometric proportion can be ordinarily had close controls are maintained.

The heat input requirements in carrying -out my calcining step will vary-with the different types of roasting furnaces commonly employed, the amount of and proportions of iron sulfate in the sinter, and thes'hadedesired in the pigment. However, for any desired pigment shade, it has been found that my improved method materially increases the calcining'rate, often to the-extent of percent or more, and thus affords a substantial economy in operation.

The fine control of the'colorof the pigment product is governed by the temperature of calcination, since "higher temperatures will give a bluish cast to the red iron' oxide. -=I am able by practicing my improved calcining -method,-when desired, to produce red iron oxide pigments having a'bluish cast and which also possess high tinting strength. Thisis quite an important advantage since, due to'the relatively high calcining temperatures required to produce a bluish cast, it has heretofore been exceedingly di-hicult, if not impossible, to obtain high tinting strength in such pigments.

By carrying 'onth'e calcination inaccordance with my'invention as hereinabove described and illustrated, I secure much higher production rates than were attained in prior commercial methods, and still theiron sulfate is substantially completely decomposedfto*ferricoxide in a substantially neutral sinter, overburning of the iron oxide as it is produced during calcination is avoided, and a high quality red iron oxide pigment, having ahightinting strength and amass tone of high chroma, is obtained. The ease of production of iron oxide pigments having light red or orange shades, as is readily possible in practicing my improvement, is one of its imortant advantages, since it is very difficult to produce orange shades (calcining to a neutral sinter) in the calcining methods of the art due to their inherent tendency to overburn the iron oxide at the temperatures and times necessary to substantially decompose the iron' sulfate; Furthermore, as previously mentioned, it has been found that the percentage of sulphur trioxide given off during the roasting, in practicing the 7 method disclosed herein, is unexpectedly lower than the. percentage given off in the roasting methods known to the art, which facilitates the recovery of sulphunas sulphuric acid by the contact method. I

While my invention has been described and illustrated hereinabove in considerable detail, it willbe'understood that the same is done by way of illustration and not by way of limitation. All embodiments and variations comprehended by the disclosure and defined in the claims, which are novel over the prior art, are comprehended.

What I claim is: 7

1. The method of producing a red iron oxide pigment of good tinting strength which comprises heating a material composed mainly of a uniform mixture of iron sulfate and carbon, the carbon being controlled in amount so that it is present in an amount within the range offrom about. 8% less than, to about 8% more than, stoichiometric; proportion with respect to the S03 content of the iron sulfate, in accordance with the equation: 2S O3+C 2SO2+CO2; said heating being carried on at a calcining temperature to convert said iron sulfate to Fezoa; the carbon in the proportions specified serving to provide uniform heating throughout the iron sulfate and a reduction; in the period of time required to calcine the iron sulfate.

2. The method of producing substantially sulfate-free and neutral red iron oxide pigments, comprising incorporating finely divided carbonaceous material into a raw material consisting predominantly of iron sulfate and calcining the resulting mixture to decompose the iron sulfate and form ironpxide without overburning, said carbon being incorporated in an amount within the-range of from about 8% less than, to about 8% more than, the stoichiometric amount required to reduce to sulfur dioxide the sulfur trioxide resulting from the decomposition of the iron sulfate, in accordance with the equation 2SO3+C=2SOa+COa 3. The method of producing substantially sulfate-free and neutral red iron oxide pigments, comprising mixing finely divided carbonaceous material with ground iron ore, treating said iron ore with strong sulfuric acid to form ferric sulfate, and then roasting the converted material to decompose the ferric sulfate and form iron oxide, said carbon being present in an amount within the range of from about 8% less than, to about 8% more than, the stoichiometric amount required to reduce to sulfur dioxide the sulfur trioxide resulting from the decomposition of the ferric sulfate, in accordance withthe equation 2SO3+C:2SO2+CO2.

4. The method of producing substantially sulfate-free and neutral red iron oxide pigments, comprising: mixing ground iron ore, carbon, concentrated sulfuric acid and water, said acid being in an amount sufficient to convert upwards of" of the ironcontent of said are to ferric sulfate, and said carbon being in an amount stoichiometrically equivalent to the sulfate content of said acid according to the equation 2SOa+C=2SOz+CO2; reacting said ore and said acid to formferric sulfate; and then calcining the resulting mixture to form a substantially neutral sinter. a a

5. In the method of producing an iron oxide pigment which comprises mixing ground iron ore with concentrated sulfuric acid in an amount less than that required to convert all of the iron oxide content of the ore to ferric sulfate, but in an amount sufficient .to convert at least about 70%-of said iron oxide content to ferric sulfate, and reacting said iron and said acid at an elevated temperature so as to form a reaction product which is fluid at said elevated temperature and .which will form agranular sensibly dry material upon cooling, and then calcining such material. toeconvert the ferric sulfate to FezOs and SO3,'..and to form an iron oxide pigment, that improvement which consists in adding/to the mixture, at some point prior to said calcining operation, an amount of carbon in finely divided form, said amount of carbon being essentially just sufficient, within the range of about 8% less than stoichiometric to about 8% more than stoichiomctric, to carry out the reaction indicated by the equation 2.SO3+C=2SO2+CO2, whereby to form a. neutral, sulfate-free red iron oxide pigment.

GEORGE E. NOPONEN.

REFERENCES CITED UNITED STATES PATENTS Number 7 Name Date 453,137 Ruymbeke May 26, 1891 1,045,723 McFetridge NOV. 26, 1912 7 1,048,247 Weeks Dec. 24, 1912 1,472,131 Maxham Oct. 30, 1923 1,489,361 Maxham Apr. 8, 1924 2,184,738 Fireman Dec. 26, 1939 2,208,905 7 Heckman June 11, 1940 2,215,394 Hechenbleikner Sept. 17, 1940 2,242,7 3 s ith May 20, 1941 2,252,332 Plummer Aug. 12, 1947 2,415,133 Ayers Feb. 18, 1947 FOREIGN PATENTS Number 7 Country Date 4,201 Great Britain Sept. 29, 1881 of 1881 523,241 Great Britain July 9, 1940 OTHER REFERENCES Berling, Oxide of Iron Pigments, Can. Chem. and Met., vol. 15, January 1931, page 18, lines 9 to 25. 

1. THE METHOD OF PRODUCING A RED IRON OXIDE PIGMENT OF GOOD TINTING STRENGTH WHICH COMPRISES HEATING A MATERIAL COMPOSED MAINLY OF A UNIFORM MIXTURE OF IRON SULFATE AND CARBON, THE CARBON BEING CONTROLLED IN AMOUNT SO THAT IT IS PRESENT IN AN AMOUNT WITHIN THE RANGE OF FROM ABOUT 8% LESS THAN, TO ABOUT 8% MORE THAN, STOICHIOMETRIC PROPORTION WITH RESPECT TO THE SO3 CONTENT OF THE IRON SULFATE, IN ACCORDANCE WITH THE EQUATION: 2SO3+C$2S02+CO2; SAID HEATING BEING CARRIED ON AT A CALCINING TEMPERATURE TO CONVERT SAID IRON SULFATE TO FE2O3; THE CARBON IN THE PROPORTIONS SPECIFIED SERVING TO PROVIDE UNIFROM HEATING THROUGHOUT THE IRON SULFATE AND A REDUCTION IN THE PERIOD OF TIME REQUIRED TO CALCINE THE IRON SULFATE. 